S.-G. Eriksson

Synthesis, structural and magnetic characterisation of the double perovskite A2MnMoO6 (A=Ba, Sr)

Abstract A study of the crystallographic structure and magnetic properties of the double perovskites Ba2MnMoO6 and Sr2MnMoO6 in polycrystalline form has been carried out by means of neutron powder diffraction (NPD) and magnetization measurements. The Rietveld analysis of room temperature data shows that the Mn2+ and Mo6+ ions are B-site ordered, i.e. the structure is a NaCl-type ordered double perovskite. Ba2MnMoO6 crystallizes in the cubic space group Fm 3 m (a=8.1680(1)) and Sr2MnMoO6 crystallizes in the space group P42/n (a=7.9575(5), c=7.9583(9)). Bond valence sum (BVS) calculation revealed that these compounds have the valency pair of {Mn2+(3d5;t32ge2g), Mo6+(4d0)}. The magnetic measurements suggest that these compounds transform to an antiferromagnetic state below 10 K.

Synthesis, crystal structure, and magnetic characterization of the double perovskite Ba2MnWO6

The double perovskite Ba2MnWO6 has been prepared as a pure powder by a conventional solid-state reaction process and studied by X-ray, neutron powder diffraction (NPD), magnetization, and AC susceptibility measurements. NPD, magnetization, and AC suscepti

Nuclear and magnetic structure of Ca2MnWO6 : A neutron powder diffraction study

Abstract The nuclear and magnetic structures of the double provskite compound Ca 2 MnWO 6 have been determined by neutron powder diffraction. Rietveld refinement shows that the compound adopts a monoclinic crystal structure with P2 1 /n symmetry. Magnetic refinement at 10 K shows that the magnetic structure is based on a unit cell related to that of the nuclear structure by a propagation vector (0 1/2 1/2), which is not very common in the case of double perovskites. The crystal contains alternating MnO 6 and WO 6 octahedra, considerably tilted due to the relative small size of the cation that occupy the A-sublattice of the perovskite. The manganese magnetic moments are coupled antiferromagnetically along the unit cell axes with the magnetic moment equal to 3.49(2) μ B (M x = 1.08(13) μ B , M y = 0.94(7) μ B , M z = 3.20(4) μ B ). Antiferromagnetic behavior was confirmed by magnetization measurements, and the antiferromagnetic to paramagnetic transition was found at 16 ± 0.1 K.

Preparation of Zn substituted Ni-Fe-Cr ferrites and study of the crystal structure by neutron diffraction

The spinel type solid solutions of ZnxNi1−xFeCrO4 (x=0.2, 0.4, 0.6 and 0.8) were obtained by solid state sintering technique in air at 1523 K. X-ray and neutron powder diffraction experiments have been carried out on the samples at 295 K in order to characterize the materials. Rietveld refinement of the neutron diffraction data reveals that all the samples of the series possess cubic symmetry corresponding to the space group Fd3m. The distribution of Zn ions on the A and B sites as affected by the variation of x and also the distribution of Cr ions on the A and B sites have been determined. The Fe ions are distributed over both the A and B sites for the whole compositional range investigated. Only a small portion of the Ni ions are found to occupy the A site for x=0.2, while all the Ni ions enter the B site for x≥0.4. The fractional coordinates of oxygen u, and the lattice parameter a, increase with increasing Zn content. The moment distributions in the two sublattices for different composition have been determined.

The study of magnetic ordering in the spinel system ZnxNi1−xFeCrO4 by neutron diffraction

Abstract Neutron diffraction measurements have been performed in the temperature range 600 K⩾ T ⩾10 K on the spinel system ZnxNi1−xFeCrO4 (x=0.2, 0.4, 0.6 and 0.8) synthesized in the ceramic sintering method. The moment distributions in the two sublattices and their ordering as affected by compositional change and temperature variation have been determined. Ferrimagnetic transition temperatures and spin compensation temperatures for all the specimens have been determined. The system shows significant deviations from the usual ferrimagnetic behavior, which increases with the increase of diamagnetic substitutions, being more prominent in the B sublattice. The ferrimagnetic ordering is perturbed due to the presence of non-collinear spins mainly in the B site. Presence of small fluctuating magnetic clusters for x⩾0.6 is evident from the diffuse nature of the scattered intensity. A qualitative explanation of the observed features is put forward in the light of competing inter and intra sublattice exchange interactions. A randomly canted ferrimagnetic ordering is suggestive for the system at x⩽0.4, while a semi-spin glass like transition is favorable for x⩾0.6 due to large reduction in moment and evolution of diffuse signal from short-range clusters at low temperatures.

Structure and properties of niobium-doped potassium titanyl phosphate crystals

A series of potassium titanyl phosphate crystals, KTiOPO4, with various concentrations of niobium dopant has been grown, and some of their physical properties and structural characteristics have been studied. The incorporation of a small amount of niobium results in considerable changes in the electrical conductivity of KTP: Nb crystals and the temperature of the ferroelectric phase transition. Thus, the presence of 3–4 at. % of niobium results in an increase of conductivity by more than an order of magnitude, whereas TC decreases from 930 to 620°C. The X-ray diffraction study of the crystals has been performed at room temperature; the neutron diffraction analysis was made at temperatures of 20, 330, and 730°C. It was revealed that two crystallographically independent positions are statistically (by 90%) occupied by potassium cations, which results in the concentration of potassium atoms in the structure higher than it was expected from the condition of preservation of crystal electroneutrality. At high niobium concentrations, the monoclinic compound of the composition K2TiNb2P2O3 is formed.

High resolution neutron powder diffraction study of the microstrain contribution in Pb(ZrxTi1-x)O3 ceramics

Abstract The high-resolution neutron powder diffraction data collected from Pb(Zr x Ti 1- x )O 3 ( x =0.20, 0.52, 0.53 and 0.54) samples between 4 and 900 K were analysed using Rietveld refinement. The high-resolution powder diffractometer (HRPD) at Rutherford Appleton Laboratory (ISIS facility) was used. Particular attention was paid to two phase coexistence in the vicinity of the morphotropic phase boundary and to the microstrain contribution. Strongly reflection indices ( h , k , l ) dependent line broadening was related to a spatial variation of x which probes the sensitivity of different crystal lattice spacings against x .

Local and Average Structure of Lead Titanate Based Ceramics

The revised lattice dynamics of Pb(Zr x Ti 1 m x )O 3 (PZT) with 0.10 h x h 0.54 ceramics is reported. The phase transition from tetragonal phase to the monoclinic phase is proposed to occur via locally distorted regions. Neutron diffraction data suggests that these local regions transform to the monoclinic phase with decreasing temperature, when x , 0.50.

Synthesis and Crystal Structure of the Double Perovskite Ca2-xSrxMnWO6 (x = 0.0, 0.5, 1.0, 1.5, 2.0)

The double perovskites Ca2-xSrxMnWO6 (x = 0.0, 0.5, 1.0, 1.5, 2.0) have been prepared as pure phases by a conventional solid-state reaction process. Stoichiometric amounts of CaCO3,SrCO3, MnO and WO3 were mixed, ground and calcined in a nitrogen atmosphere. X-ray and neutron powder diffraction (NPD), together with magnetisation measurements were carried out in order to characterise the materials. The NPD and magnetisation measurements, performed at different temperatures, were made in order to correlate the materials structural and magnetic properties. Susceptibility- Temperature (ϰ-T) curves were measured between 2K and 200K in field cooled (FC) and zero-field-cooled (ZFC) modes with an applied field (H) up to 3T. Rietveld analysis of NPD data shows that the samples were B-site ordered perovskites with different types of lattice symmetry. Magnetisation measurements at low temperatures indicate an antiferromagnetic ordering of the magnetic moments.

Synthesis, Cation Distribution and Crystal Structure of the Spinel System ZnxNi1-xFeCrO4 (0.2 ≤ x ≤ 0.8)

The spinel system ZnxNi1-xFeCrO4 (x = 0.2, 0.4, 0.6 & 0.8) has been prepared by solid state reaction in the standard ceramic sintering method. X-ray and neutron powder diffraction experiments have been carried out on the samples at 295K. From the diffraction patterns all the samples of the series have been found to possess cubic symmetry corresponding to the space group Fd-3m. The distribution of the four cations Zn, Ni, Fe and Cr over the two sublattices, lattice parameter and oxygen position parameter have been determined precisely. The system has been found to be ferrimagnetic at room temperature.